1. Field of the Invention
The present invention relates to a BTL (Balanced Transformer Less) amplifier apparatus for driving a speaker.
2. Description of the Related Art
Generally, the BTL amplifier has various advantages: (1) a high output is obtained from a low power supply voltage, (2) distortion at even order is canceled; (3) the amount of ripple rejection of the power supply is much. For this reason, it has been widely used in an audio system for a vehicle.
FIG. 9 shows a typical conventional BTL amplifier 1 formed in an integrated circuit. The BTL amplifier 1 includes an inverting amplifier 11 for inverting an input signal and amplifiers 13 and 14 each having a gain equal to that of the inverting amplifier 11.
Reference numeral 2 denotes an electronic volume, 3 a coupling capacitor and 4 a speaker.
Nowadays, the BTL amplifier 1 and electronic volume 2 are designed in a semiconductor integrated circuit in which they are DC-coupled with a capacitor and coil removed in order to maintain the preferred characteristic in a low frequency range and high frequency range.
Therefore, there is a DC potential difference between the output of the electronic volume 2 and an input of the BTL amplifier 1. If the electronic volume 2 and BTL amplifier 1 are connected to each other with such a potential difference, the respective circuits do not operate normally. In order to obviate such an inconvenience, a capacitor 3 is connected for coupling the output from the electronic volume 2 with the BTL amplifier.
However, if the capacitor 3 is deteriorated to generate a leak current and the BTL amplifier 1 is deteriorated, a DC current flows through a voice coil of a speaker so that the voice coil may be damaged by the generated distortion.